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Cold neutron source efficiency in a tangetial beam hole
NUCLEAR
1N57RUMEN75
AND
96 (1971) 2 8 5 - 2 8 8 ; •
ME7H0D5
C0LD NEU7R0N
50URCE
N0R7H-H0LLAND
EFF1C1ENCY 1N A 7 A N 6 E N 7 1 A L
PU8L15H1N6
C0.
8EAM H0LE
V. D1M1C a n d J. P E 7 K 0 V ~ E K
1n5t1tude ~J0~ef 5tefan••, Lju61jana, Yu9051av1a Rece1ved 11 M a y 1971 C01d n e u t r 0 n 6 e a m 1nten51ty ha5 6een mea5ured f0r d1fferent1y 5haped m 0 d e r a t 0 r cham6er5 at d1fferent p051t10n5 1n a 15 cm d1ameter tan9ent1a1 6 e a m h01e 0 f the react0r 7r19a M a r k 11. Mea5urement5 have 6een p e r f 0 r m e d at am61ent temperature5.
1t ha5 6een f 0 u n d that the m05t fav0ura61e c0nf19urat10n f0r a m 0 d e r a t 0 r 15 a ••61ack 60dy•• 5catterer w1th a 9a1n fact0r 0 f 19 re1at1ve t0 the e m p t y 6 e a m h01e a n d 0 f 1.7 re1at1ve t0 the 15 c m 10n9 9raph1te 51a6.
1. 1ntr0duct10n
50urce. 51nce then many mater1a15 have 6een u5ed a5 m0derat0r5. L14u1d hydr09en and methane have the 6e5t m0derat1n9 pr0pert1e5 6ecau5e 0f the1r 5ma11 510w1n9 d0wn 1en9th and h19h 5catter1n9 cr055 5ect10n, h0wever, they are 1n a 9a5e0u5 5tate at N7P.
1t 15 n0w very we11 e5ta6115hed that c01d neutr0n 5pectr05c0py 15 an 1mp0rtant t001 1n 5tudy1n9 the dynam1ca1 pr0pert1e5 0f 5011d5 and 114u1d5, 6ut 1nten51t1e5 0f c01d neutr0n5 w1th ener91e5 1e55 than 5 meV are u5ua11y 10w. 1t wa5 5u99e5ted 6y E9e15taff et a1.1) that 114u1d hydr09en 51tuated near the centre 0f a react0r c0u1d 6e u5ed a5 a c0nven1ent c01d neutr0n
] 10•
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-
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a~
neutr~n
1n 1mn1m111~
F19. 1. C0unt1n9 rate5 f0r d1fferent c1rcu1ar a n d e111pt1ca1 51a65 a5 funct10n 0 f the p051t10n 1.
285
F19. 2. C0unt1n9 rate5 f0r d1fferent 61ack 6 0 d y 5catterer5 a5 funct10n 0 f the p051t10n 1.
286
V. D1M1C A N D J. P E 7 K 0 V 5 E K
An0ther 1mp0rtant fact0r 90vern1n9 the 1nten51ty 0f the c01d neutr0n 50urce 15 the 5hape 0f the 50urce cham6er. We662) pr0p05ed 50me 5catterer 5hape5 t0 1mpr0ve the 1nten51t1e5. A 5y5temat1c exper1menta1 5tudy 0f d1fferent d1men510n5 0f the 50urce cham6er 1n a rad1a1 react0r 6eam tu6e ha5 6een carr1ed 0ut 1n V1enna3). 50me 5urvey mea5urement5 0n d1fferent1y 5haped 1uc1te 5amp1e5 have 6een perf0rmed 1n a 9raph1te 5tack wh1ch 51mu1ated a tan9ent1a1 6eam h01e 0f a react0r4). W1th th15 exper1menta1 5etup 1t 15 n0t p055161e t0 9et an an5wer a60ut the 0pt1mum p051t10n 0f the m0derat0r cham6er re1at1ve t0 the react0r c0re. 1n 0rder t0 an5wer th15 4ue5t10n and at the 5ame t1me t0 f1nd the 0pt1mum 5catterer 5hape f0r the p1anned c01d neutr0n 50urce at 0ur react0r 1t wa5 dec1ded t0 perf0rm an exper1ment 0n 0pt1m15at10n 0f a c01d neutr0n 50urce 1n the tan9ent1a1 6eam h01e 0f the react0r 7r19a Mark 11. 1n 5ect10n 2 the exper1menta1 arran9ement 15 de5cr16ed, wh11e 5ect10n 3 0ut11ne5 the re5u1t5.
70 1
2. De5cr1pt10n 0f the exper1ment 7he t1me 0f f119ht 5pectr0meter at the tan9ent1a1 6eam h01e 0f the 250 kW 7r19a react0r 15 de519ned f0r c01d neutr0n 5catter1n9 exper1ment55). 7h15 5pectr0meter ha5 6een u5ed f0r the ana1y515 0f the emer91n9 neutr0n 6eam. 7hree d1fferent m0derat0r cham6er5 w1th d1fferent d1men510n5 have 6een ch05en [f0110w1n9 ref. 4]: a) c1rcu1ar 51a6, 6) e111pt1ca151a6 and c) 61ack 60dy 5catterer. 7he c01d neutr0n 0utput f01"a11 d1fferent cham6er5 ha5 6een mea5ured at d1fferent p051t10n5 re1at1ve t0 the centre 11ne 0f the react0r c0re. W1th + 1 we have den0ted the d15tance fr0m the centre 11ne t0 the cham6er 1n the d1rect10n 0f the emer91n9 neutr0n 6eam and w1th - 1 1n the 0pp051te d1rect10n. Methy1a1c0h01 ha5 6een taken a5 the 5catter1n9 mater1a1 ma1n1y 6ecau5e 0f 1t5 114u1d 5tate at N 7 P and 6ecau5e 0f 1t5 900d m0derat1n9 pr0pert1e56).
3. Exper1menta1 re5u1t5 and d15cu5510n 1t ha5 6een f0und that mea5urement5 0n the 0pt1m1-
*10~c°unt5110~
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F19. 3. C0unt1n9 rate5 f0r d1fferent 61ack 60dy 5catterer5 a5 funct10n 0fthe 5he11th1ckne55d.
C0LD
NEU7R0N
50URCE
EFF1C1ENCY
5at10n 0 f the 5catterer 5hape can 6e m a d e at r00m temperature4). 1f the c01d neutr0n 0 u t p u t at r 0 0 m t e m p e r a t u r e 15 the h19he5t f0r a certa1n 5catterer, then th15 5catterer 91ve5 the 1ar9e5t 0 u t p u t a150 at 10w temperature5. 7heref0re, a11 mea5urement5 0 f d1fferent1y 5haped m 0 d e r a t 0 r cham6er5 a n d at d1fferent p051t10n5 have 6een p e r f 0 r m e d at an am61ent t e m p e r a t u r e . 7 h e 1nte9ra1 1nten51ty 6e10w the 6ery1[1um cut-0ff ha5 6een t a k e n a5 the mea5ure 0 f the c01d neutr0n 0utput. 1n 5uch a ca5e 1t 15 very ea5y t0 e5t1mate the 6 a c k 9 r 0 u n d a n d a11 5pectra were c0rrected f0r 1t. 7 h e d15tance 1 re1at1ve t0 the centre 0 f the c0re f0r a1m05t a11 m 0 d e r a t 0 r cham6er5 ha5 6een var1ed fr0m + 10 cm t0 - 3 0 cm, 1n 5tep5 0f 10 cm. Mea5urement5 have 6een p e r f 0 r m e d 0n the f0110w1n9 m 0 d e r a t 0 r cham6er5:
7A8LE
81ack 60dy
d (cm)
H0LE
/0pt (cm)
1 C0unt1n9 rate (ct5/10•) × 103
91
92
3
1
-- 5
60.0
15.8
1.45
5
1 2
-- 8 --8
60.5 59.5
15.9 15.6
1.46 1.43
3
--8
56.0
14.7
1.35
1 2 3
- 10 - 10 - 10
69.0 71.5 58.0
18.1 18.8 15.2
1.66 1.72 1.40
1 3
---
-- 1 - 3
47.0 55.0
12.3 14.5
1.13 1.32
5 10
---
--5 -- 10
55.5 56.0
14.6 14.7
1.34 1.35
tran5m15510n
7
-
- 3
46.5
12.2
1.12
E111pt1ca1 51a6 1n ref1ect10n
7
--
--6
63.5
16.7
1.53
15
--
--
41.5
10.9
1.0
--
--
--
3.8
1.0
--
C1rcu1ar 51a6
E111pt1ca1 51a6 1n
6raph1te W1th0ut
5catterer
287
3.3. 8LACK 80DY 5CA77ERER W1th a ••61ack 60dy•• we mean the m 0 d e r a t 0 r c h a m 6 e r cy11ndr1ca1 1n 5hape w1th a deep re-entry h01e 50 that the emer91n9 neutr0n5 are 1n therma1 e4u1116r1um a5 much a5 p055161e at the m 0 d e r a t 0 r temperature. 7 h e 6 0 t t 0 m 0 f t h e c h a m 6 e r 15 a c1rcu1ar 51a6 w1th the f0110w1n9 d1men510n5: a = 3 c m , 5cm and 7cm. 7 h e cy11ndr1ca1 5he11 ha5 th1ckne55e5 0f d = 1 cm, 2 cm and 3 cm w1th the 1nner d1ameter 0f 14.8 cm. 7 h e 0vera11 1en9th 0f the c h a m 6 e r 15 30 cm (f19.2). 7 h e d15tance 115 mea5ured fr0m the m1d-p01nt 0f the c1rcu1ar 51a6. 7 h e re5u1t5 are 5h0wn 1n f195. 2 a n d 3. C0mpar150n 0f f195. 1 a n d 2 5h0w5 c1ear1y the mer1t5 0f the 61ack 60dy 5catterer. 7 h e c h a m 6 e r w1th d1men510n5: a = 7 cm, d = 1.6 cm at
We have Ch05en the C1rcu1ar 51a65 w1th the d1ameter 0f 14.8 Cm and the f0110w1n9 th1Ckne55e5: a = 1 Cm, 3Cm, 5Cm a n d 1 0 c m . 7 h e d15tance 1 ha5 6een mea5Ured fr0m the m1dd1e 0 f the c1rCU1ar 51a65. 7 h e re5u1t5 are 5h0Wn 1n f19. 1.1t 15 5een t h a t an 0pt1mum th1Ckne55 0 f a 51a6 15 10 Cm at the p051t10n 1 = -- 10 Cm. A1m05t the 5ame C01d neutr0n 0utpUt ha5 6een f0Und f0r th1Ckne55e5 0 f 3 cm a n d 5 cm at the p051t10n 1=--3Cm and1=--5cm.
a (cm)
8EAM
3.2. E L L 1 P 7 1 C A L 5 L A 8 5 7 h e 5ymmetry ax15 0 f the 7 cm th1ck e111pt1ca1 51a6 wa5 1nc11ned t0ward5 the wa11 0 f the 6eam h01e f0r a 6 0 u t ~0 = 25 °. Mea5urement5 have 6een p e r f 0 r m e d 1n ref1ect1n9 a n d tran5m1tt1n9 p051t10n5 (f19. 1), 1 ha5 6een mea5ured fr0m the m1d-p01nt 0 f the cham6er. 7 h e 6e5t c01d neutr0n 0 u t p u t ha5 6een f0und f0r an e111pt1ca1 51a6 1n a ref1ect1n9 p051t10n at 1 = - 6 cm a n d 1t 15 f0r a 6 0 u t 10% 6etter a5 the 6e5t neutr0n 0 u t p u t fr0m a c1rcu1ar 51a6. Mea5urement5 have 6een m a d e w1th a n 0 t h e r e111pt1ca1 51a6 at ~p = 45 °, 6ut the re5u1t5 have 6een a1m05t the 5ame a5 f0r c1rcu1ar 51a65.
3.1. C1RCULAR5LA85
5catterer
1N A 7AN6EN71AL
288
v. D1M1C A N D J. P E 7 K 0 V 5 E K
1 = - 10 cm ha5 a part1cu1ar1y fav0ura61e 9e0metry. [n th15 ca5e an 1ncrea5e 0f the c01d neutr0n 1nten51ty 15 a1m05t f0r a fact0r 0f 20 c0mpared t0 an empty 6eam h01e. When 9raph1te ha5 6een u5ed a5 the reference 5catterer, a 9a1n fact0r 0f 1.7 ha5 6een 06ta1ned. A11 re5u1t5 0f the mea5urement5 0n the 0pt1m15at10n 0f the m0derat0r 5hape and p051t10n are 91ven 1n ta61e 1 where tw0 9a1n fact0r5 91 and #2 have 6een 1ntr0duced. 7he 9a1n fact0r 91 15 def1ned a5 the rat10 0f the c0rrected c01d neutr0n 0utput re1at1ve t0 the empty 6eam h01e and 9 2 a5 the rat10 0f the c0rrected c01d neutr0n 0utput re1at1ve t0 the 0utput fr0m the 9raph1te 51a6 0fthe 1en9th 15 cm and d1ameter 14.8 cm. 7he d15tance /0pt 15 the 0pt1mum p051t10n 0f d1fferent m0derat0r cham6er5. C0mpar150n w1th prev10u51y rep0rted w0rk at 15pra 4) 15 0n1y p055161e w1th the re5u1t5 at the / = 0 p051t10n 0f a m0derat0r cham6er. 1n th15
ca5e, the a9reement 6etween 60th mea5urement515 4u1te 900d. We can c0nc1ude that the 5hape and p051t10n 0f a m0derat0r cham6er 1n a tan9ent1a1 6eam h01e are very 1mp0rtant feature5 6ecau5e the c01d neutr0n 0utput 15 very dependent 0n 60th character15t1c5. We 5h0u1d 11ke t0 expre55 0ur 9rat1tude t0 Dr. M. 120p1E f0r u5efu1 5u99e5t10n5 and d15cu5510n5. Reference5 1) p. A. E9e15taff, H. L0nd0n and F. J. We66, C0nf. Phy514ue de5 6a55e5 temperature5 (Par15, 1955). •) F. J. We66, React0r 5c1. 7echn01. 17 (1963) 187. a) H. Rauch and H. 5chm1dt, At0mkernener91e 10-39 (1965) 243.
4) L. 011v1and R. M15enta, EUR 2953-e (1966). 5) V. D1m1cand J. Petk0v~ek, N1J5 Rep0rt R-515 (1967).
1N57RUMEN75
AND
96 (1971) 2 8 5 - 2 8 8 ; •
ME7H0D5
C0LD NEU7R0N
50URCE
N0R7H-H0LLAND
EFF1C1ENCY 1N A 7 A N 6 E N 7 1 A L
PU8L15H1N6
C0.
8EAM H0LE
V. D1M1C a n d J. P E 7 K 0 V ~ E K
1n5t1tude ~J0~ef 5tefan••, Lju61jana, Yu9051av1a Rece1ved 11 M a y 1971 C01d n e u t r 0 n 6 e a m 1nten51ty ha5 6een mea5ured f0r d1fferent1y 5haped m 0 d e r a t 0 r cham6er5 at d1fferent p051t10n5 1n a 15 cm d1ameter tan9ent1a1 6 e a m h01e 0 f the react0r 7r19a M a r k 11. Mea5urement5 have 6een p e r f 0 r m e d at am61ent temperature5.
1t ha5 6een f 0 u n d that the m05t fav0ura61e c0nf19urat10n f0r a m 0 d e r a t 0 r 15 a ••61ack 60dy•• 5catterer w1th a 9a1n fact0r 0 f 19 re1at1ve t0 the e m p t y 6 e a m h01e a n d 0 f 1.7 re1at1ve t0 the 15 c m 10n9 9raph1te 51a6.
1. 1ntr0duct10n
50urce. 51nce then many mater1a15 have 6een u5ed a5 m0derat0r5. L14u1d hydr09en and methane have the 6e5t m0derat1n9 pr0pert1e5 6ecau5e 0f the1r 5ma11 510w1n9 d0wn 1en9th and h19h 5catter1n9 cr055 5ect10n, h0wever, they are 1n a 9a5e0u5 5tate at N7P.
1t 15 n0w very we11 e5ta6115hed that c01d neutr0n 5pectr05c0py 15 an 1mp0rtant t001 1n 5tudy1n9 the dynam1ca1 pr0pert1e5 0f 5011d5 and 114u1d5, 6ut 1nten51t1e5 0f c01d neutr0n5 w1th ener91e5 1e55 than 5 meV are u5ua11y 10w. 1t wa5 5u99e5ted 6y E9e15taff et a1.1) that 114u1d hydr09en 51tuated near the centre 0f a react0r c0u1d 6e u5ed a5 a c0nven1ent c01d neutr0n
] 10•
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1
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-
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1n 1mn1m111~
F19. 1. C0unt1n9 rate5 f0r d1fferent c1rcu1ar a n d e111pt1ca1 51a65 a5 funct10n 0 f the p051t10n 1.
285
F19. 2. C0unt1n9 rate5 f0r d1fferent 61ack 6 0 d y 5catterer5 a5 funct10n 0 f the p051t10n 1.
286
V. D1M1C A N D J. P E 7 K 0 V 5 E K
An0ther 1mp0rtant fact0r 90vern1n9 the 1nten51ty 0f the c01d neutr0n 50urce 15 the 5hape 0f the 50urce cham6er. We662) pr0p05ed 50me 5catterer 5hape5 t0 1mpr0ve the 1nten51t1e5. A 5y5temat1c exper1menta1 5tudy 0f d1fferent d1men510n5 0f the 50urce cham6er 1n a rad1a1 react0r 6eam tu6e ha5 6een carr1ed 0ut 1n V1enna3). 50me 5urvey mea5urement5 0n d1fferent1y 5haped 1uc1te 5amp1e5 have 6een perf0rmed 1n a 9raph1te 5tack wh1ch 51mu1ated a tan9ent1a1 6eam h01e 0f a react0r4). W1th th15 exper1menta1 5etup 1t 15 n0t p055161e t0 9et an an5wer a60ut the 0pt1mum p051t10n 0f the m0derat0r cham6er re1at1ve t0 the react0r c0re. 1n 0rder t0 an5wer th15 4ue5t10n and at the 5ame t1me t0 f1nd the 0pt1mum 5catterer 5hape f0r the p1anned c01d neutr0n 50urce at 0ur react0r 1t wa5 dec1ded t0 perf0rm an exper1ment 0n 0pt1m15at10n 0f a c01d neutr0n 50urce 1n the tan9ent1a1 6eam h01e 0f the react0r 7r19a Mark 11. 1n 5ect10n 2 the exper1menta1 arran9ement 15 de5cr16ed, wh11e 5ect10n 3 0ut11ne5 the re5u1t5.
70 1
2. De5cr1pt10n 0f the exper1ment 7he t1me 0f f119ht 5pectr0meter at the tan9ent1a1 6eam h01e 0f the 250 kW 7r19a react0r 15 de519ned f0r c01d neutr0n 5catter1n9 exper1ment55). 7h15 5pectr0meter ha5 6een u5ed f0r the ana1y515 0f the emer91n9 neutr0n 6eam. 7hree d1fferent m0derat0r cham6er5 w1th d1fferent d1men510n5 have 6een ch05en [f0110w1n9 ref. 4]: a) c1rcu1ar 51a6, 6) e111pt1ca151a6 and c) 61ack 60dy 5catterer. 7he c01d neutr0n 0utput f01"a11 d1fferent cham6er5 ha5 6een mea5ured at d1fferent p051t10n5 re1at1ve t0 the centre 11ne 0f the react0r c0re. W1th + 1 we have den0ted the d15tance fr0m the centre 11ne t0 the cham6er 1n the d1rect10n 0f the emer91n9 neutr0n 6eam and w1th - 1 1n the 0pp051te d1rect10n. Methy1a1c0h01 ha5 6een taken a5 the 5catter1n9 mater1a1 ma1n1y 6ecau5e 0f 1t5 114u1d 5tate at N 7 P and 6ecau5e 0f 1t5 900d m0derat1n9 pr0pert1e56).
3. Exper1menta1 re5u1t5 and d15cu5510n 1t ha5 6een f0und that mea5urement5 0n the 0pt1m1-
*10~c°unt5110~
60
"~, / /.1
J
7
"•" --. - 1 0
f j
50 .J
40 ] 1 0
a
=
7crn
a
=
5crn
1 2
1 3
••10 =. d [ernJ
F19. 3. C0unt1n9 rate5 f0r d1fferent 61ack 60dy 5catterer5 a5 funct10n 0fthe 5he11th1ckne55d.
C0LD
NEU7R0N
50URCE
EFF1C1ENCY
5at10n 0 f the 5catterer 5hape can 6e m a d e at r00m temperature4). 1f the c01d neutr0n 0 u t p u t at r 0 0 m t e m p e r a t u r e 15 the h19he5t f0r a certa1n 5catterer, then th15 5catterer 91ve5 the 1ar9e5t 0 u t p u t a150 at 10w temperature5. 7heref0re, a11 mea5urement5 0 f d1fferent1y 5haped m 0 d e r a t 0 r cham6er5 a n d at d1fferent p051t10n5 have 6een p e r f 0 r m e d at an am61ent t e m p e r a t u r e . 7 h e 1nte9ra1 1nten51ty 6e10w the 6ery1[1um cut-0ff ha5 6een t a k e n a5 the mea5ure 0 f the c01d neutr0n 0utput. 1n 5uch a ca5e 1t 15 very ea5y t0 e5t1mate the 6 a c k 9 r 0 u n d a n d a11 5pectra were c0rrected f0r 1t. 7 h e d15tance 1 re1at1ve t0 the centre 0 f the c0re f0r a1m05t a11 m 0 d e r a t 0 r cham6er5 ha5 6een var1ed fr0m + 10 cm t0 - 3 0 cm, 1n 5tep5 0f 10 cm. Mea5urement5 have 6een p e r f 0 r m e d 0n the f0110w1n9 m 0 d e r a t 0 r cham6er5:
7A8LE
81ack 60dy
d (cm)
H0LE
/0pt (cm)
1 C0unt1n9 rate (ct5/10•) × 103
91
92
3
1
-- 5
60.0
15.8
1.45
5
1 2
-- 8 --8
60.5 59.5
15.9 15.6
1.46 1.43
3
--8
56.0
14.7
1.35
1 2 3
- 10 - 10 - 10
69.0 71.5 58.0
18.1 18.8 15.2
1.66 1.72 1.40
1 3
---
-- 1 - 3
47.0 55.0
12.3 14.5
1.13 1.32
5 10
---
--5 -- 10
55.5 56.0
14.6 14.7
1.34 1.35
tran5m15510n
7
-
- 3
46.5
12.2
1.12
E111pt1ca1 51a6 1n ref1ect10n
7
--
--6
63.5
16.7
1.53
15
--
--
41.5
10.9
1.0
--
--
--
3.8
1.0
--
C1rcu1ar 51a6
E111pt1ca1 51a6 1n
6raph1te W1th0ut
5catterer
287
3.3. 8LACK 80DY 5CA77ERER W1th a ••61ack 60dy•• we mean the m 0 d e r a t 0 r c h a m 6 e r cy11ndr1ca1 1n 5hape w1th a deep re-entry h01e 50 that the emer91n9 neutr0n5 are 1n therma1 e4u1116r1um a5 much a5 p055161e at the m 0 d e r a t 0 r temperature. 7 h e 6 0 t t 0 m 0 f t h e c h a m 6 e r 15 a c1rcu1ar 51a6 w1th the f0110w1n9 d1men510n5: a = 3 c m , 5cm and 7cm. 7 h e cy11ndr1ca1 5he11 ha5 th1ckne55e5 0f d = 1 cm, 2 cm and 3 cm w1th the 1nner d1ameter 0f 14.8 cm. 7 h e 0vera11 1en9th 0f the c h a m 6 e r 15 30 cm (f19.2). 7 h e d15tance 115 mea5ured fr0m the m1d-p01nt 0f the c1rcu1ar 51a6. 7 h e re5u1t5 are 5h0wn 1n f195. 2 a n d 3. C0mpar150n 0f f195. 1 a n d 2 5h0w5 c1ear1y the mer1t5 0f the 61ack 60dy 5catterer. 7 h e c h a m 6 e r w1th d1men510n5: a = 7 cm, d = 1.6 cm at
We have Ch05en the C1rcu1ar 51a65 w1th the d1ameter 0f 14.8 Cm and the f0110w1n9 th1Ckne55e5: a = 1 Cm, 3Cm, 5Cm a n d 1 0 c m . 7 h e d15tance 1 ha5 6een mea5Ured fr0m the m1dd1e 0 f the c1rCU1ar 51a65. 7 h e re5u1t5 are 5h0Wn 1n f19. 1.1t 15 5een t h a t an 0pt1mum th1Ckne55 0 f a 51a6 15 10 Cm at the p051t10n 1 = -- 10 Cm. A1m05t the 5ame C01d neutr0n 0utpUt ha5 6een f0Und f0r th1Ckne55e5 0 f 3 cm a n d 5 cm at the p051t10n 1=--3Cm and1=--5cm.
a (cm)
8EAM
3.2. E L L 1 P 7 1 C A L 5 L A 8 5 7 h e 5ymmetry ax15 0 f the 7 cm th1ck e111pt1ca1 51a6 wa5 1nc11ned t0ward5 the wa11 0 f the 6eam h01e f0r a 6 0 u t ~0 = 25 °. Mea5urement5 have 6een p e r f 0 r m e d 1n ref1ect1n9 a n d tran5m1tt1n9 p051t10n5 (f19. 1), 1 ha5 6een mea5ured fr0m the m1d-p01nt 0 f the cham6er. 7 h e 6e5t c01d neutr0n 0 u t p u t ha5 6een f0und f0r an e111pt1ca1 51a6 1n a ref1ect1n9 p051t10n at 1 = - 6 cm a n d 1t 15 f0r a 6 0 u t 10% 6etter a5 the 6e5t neutr0n 0 u t p u t fr0m a c1rcu1ar 51a6. Mea5urement5 have 6een m a d e w1th a n 0 t h e r e111pt1ca1 51a6 at ~p = 45 °, 6ut the re5u1t5 have 6een a1m05t the 5ame a5 f0r c1rcu1ar 51a65.
3.1. C1RCULAR5LA85
5catterer
1N A 7AN6EN71AL
288
v. D1M1C A N D J. P E 7 K 0 V 5 E K
1 = - 10 cm ha5 a part1cu1ar1y fav0ura61e 9e0metry. [n th15 ca5e an 1ncrea5e 0f the c01d neutr0n 1nten51ty 15 a1m05t f0r a fact0r 0f 20 c0mpared t0 an empty 6eam h01e. When 9raph1te ha5 6een u5ed a5 the reference 5catterer, a 9a1n fact0r 0f 1.7 ha5 6een 06ta1ned. A11 re5u1t5 0f the mea5urement5 0n the 0pt1m15at10n 0f the m0derat0r 5hape and p051t10n are 91ven 1n ta61e 1 where tw0 9a1n fact0r5 91 and #2 have 6een 1ntr0duced. 7he 9a1n fact0r 91 15 def1ned a5 the rat10 0f the c0rrected c01d neutr0n 0utput re1at1ve t0 the empty 6eam h01e and 9 2 a5 the rat10 0f the c0rrected c01d neutr0n 0utput re1at1ve t0 the 0utput fr0m the 9raph1te 51a6 0fthe 1en9th 15 cm and d1ameter 14.8 cm. 7he d15tance /0pt 15 the 0pt1mum p051t10n 0f d1fferent m0derat0r cham6er5. C0mpar150n w1th prev10u51y rep0rted w0rk at 15pra 4) 15 0n1y p055161e w1th the re5u1t5 at the / = 0 p051t10n 0f a m0derat0r cham6er. 1n th15
ca5e, the a9reement 6etween 60th mea5urement515 4u1te 900d. We can c0nc1ude that the 5hape and p051t10n 0f a m0derat0r cham6er 1n a tan9ent1a1 6eam h01e are very 1mp0rtant feature5 6ecau5e the c01d neutr0n 0utput 15 very dependent 0n 60th character15t1c5. We 5h0u1d 11ke t0 expre55 0ur 9rat1tude t0 Dr. M. 120p1E f0r u5efu1 5u99e5t10n5 and d15cu5510n5. Reference5 1) p. A. E9e15taff, H. L0nd0n and F. J. We66, C0nf. Phy514ue de5 6a55e5 temperature5 (Par15, 1955). •) F. J. We66, React0r 5c1. 7echn01. 17 (1963) 187. a) H. Rauch and H. 5chm1dt, At0mkernener91e 10-39 (1965) 243.
4) L. 011v1and R. M15enta, EUR 2953-e (1966). 5) V. D1m1cand J. Petk0v~ek, N1J5 Rep0rt R-515 (1967).